Components based on BAW resonators play a role in particular as filters in end devices for mobile telecommunications.
Components with BAW resonators are generally constructed on a carrier substrate, e.g., a crystal wafer. Arranged between each resonator and the carrier substrate is either a recess that is filled with air, and that represents a high increase in impedance acting highly reflectively on the acoustic wave, or an acoustic mirror. In each case, this is how the acoustic wave is prevented from exiting the resonator in the direction of the carrier substrate TS.
An acoustic mirror comprises, e.g., an alternating sequence of layers with high and low acoustic impedance. The thickness of each of the mirror layers is approximately ¼ of the wavelength of the acoustic wave in the given material. A first acoustic mirror arranged under the resonator comprises an alternating sequence of layers with high and low acoustic impedance.
During production of a BAW resonator, as a rule the layers are produced on the component on top of one another, but each is produced separately from the other and when needed structured individually. The method for depositing and structuring the layers becomes more complex as the number of layers increases. For instance, errors in the deposition of the correct thickness for each layer can accumulate so that there is a significant scattering of the resonance frequencies of the resonators over the entire wafer and thus there is a significant scattering of the mid-frequency of filters.
In addition to the aforesaid BAW resonators, other components working with thin piezoelectric layers as functional layers are known, in particular thin film sensors and piezoelectric actuators. In the case of these components, as well, they are generally applied to carrier substrates and include a multilayer structure made of at least a first electrode, the piezoelectric functional layer, and a second electrode. Especially in the case of the piezoelectric actuators, multilayer structures are required in which the layer sequence of piezoelectric functional layer and electrode layer is repeated several times. In all components with a piezoelectric functional layer, the quality of the component is largely determined by the quality of the piezoelectric layer.
Realizing BAW resonators with low losses and a high piezoelectric coupling coefficient requires the strictly ordered orientation of the individual crystallites within the piezoelectric layer, which is only possible using a suitable deposition method, as a rule the PVD method, and on a suitable base. For BAW resonators, optimum electrical properties can be attained when the thin film grows such that within each crystallite of the thin film a preferred crystallographic direction is in a clear geometric relationship to a direction of the substrate. For instance, it is advantageous when the piezoelectric main axis is oriented strictly parallel to the normal on the substrate surface and the crystallites consequently grow strictly parallel to one another.
In addition to the deposition parameters, which are to be adjusted appropriately, this can in particular be attained using the selection of a suitable growth layer on which the crystallites of the piezoelectric layer can grow in an ordered manner. For this, in known components the piezoelectric layers are deposited on an electrode layer that also assumes the function of the growth layer. However, it is disadvantageous that the selection of suitable materials that offer both adequate electrode properties and also good conditions for ordered growth of the piezoelectric layer is very limited. The only materials that are available require compromises in terms of the electrode properties and the properties that support the growth of the piezoelectric layer.